The invention relates to methods, equipment and software products for non-invasively determining a hearing threshold of a test subject, by recording one or more brain responses to one or more acoustic stimuli. Non-restrictive examples of the brain responses include electroencephalography (EEG) responses and magnetoencephalography (MEG) responses, commonly referred to as “brainwaves”. The invention can be used in diagnostic and non-diagnostic applications.
A general problem in techniques which involve recording brainwave responses to an acoustic stimulus is that the brain response to a single acoustic stimulus is difficult to isolate from a huge variety of simultaneous brainwaves, most of which are unrelated to the acoustic stimulus. Furthermore, the relationship between the measured brain activity and the hearing threshold is not straightforward. In addition the commonly employed auditory brain stem responses reflect auditory processing at a rather low level in the central nervous system and may not provide information on auditory processes going on at higher levels, such as the auditory cortex.
US patent application no 2005/0018858 (Michael Sasha John) discloses various techniques for screening, threshold and diagnostic for evaluation of a patient's hearing. The techniques disclosed by John include acoustically presenting a modulated noise stimulus at a specific intensity to the patient's ear; recording response data related to the patient's response to the stimulus; performing signal analysis on the response data to generate result data; and evaluating the result data using at least one statistical technique to determine the presence of at least one auditory steady-state response. John proposes using EEG data as the response data. He proposes on-off modulation of a stimulus which contains an increasing and/or decreasing ramp section. The on-off modulation of the ramping stimulus is essentially equivalent to subjecting the test subject to several consecutive stimuli, wherein the intensity of one stimulus is higher or lower than the previous stimulus. John also proposes isolating the relevant brain response from other simultaneous brainwaves (which John calls “EEG noise”) by processing multiple stimulus-response cycles via complex statistical procedures.
A specific problem in the techniques disclosed by John is that the complex statistical procedures consume computational resources. Also, the technique proposed by John requires two nested levels of repetition, which also consumes time and computational resources. One level of repetition stems from the fact that even at a constant intensity, a stimulus-response cycle should be repeated and the time-aligned responses should be averaged to suppress the EEG noise. The other level of repetition is caused by the on-off modulation of the increasing and/or decreasing ramp. Such consumption of time is a burden on the test subject and test personnel. Prolonged testing may influence the test subjects' hearing and falsify the obtained results.